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AQA A Level Physics > Thermal Physics (NEED TO ADD RP) > Flashcards

Thermal Physics (NEED TO ADD RP) Flashcards

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1
Q

Internal energy

A

The sum of the randomly distributed kinetic energies and potential energies of the particles in a body

KE + PE

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2
Q

What causes the kinetic energy in the molecules all substances

A

The speed of the molecules

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3
Q

What causes the potential energy in the molecules of all substances

A

The separation between the molecules

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4
Q

What does the amount of KE and PE a substance contains depend on

A

Its phase of matter (solid, liquid or gas)

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5
Q

Why do all particles have different speeds and separations

A

The particles are randomly distributed

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6
Q

What is the internal energy of a system determined by

A

Temperature - high temp = high KE
Random motion of molecules
Phase - gases have highest internal energy
Intermolecular interactions between particles

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7
Q

How can the internal energy of a system be increased

A

Doing work on it
Adding thermal energy to it (heating it)

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8
Q

How can the internal energy of a system be decreased

A

Losing thermal energy to its surroundings
Changing state from a gas to a liquid or liquid to solid

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9
Q

During a change of state what happens to the different energies of the particles

A

The potential energies of the particle ensemble are changing but NOT the kinetic energies

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10
Q

First law of thermodynamics

A

The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it

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11
Q

Which situations does the first law of thermodynamics apply to

A

ALL situations not only gases

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12
Q

What happens to the internal energy of the gas and why, when a gas expands

A

Work is done by the gas on the surroundings and this decreases the internal energy of the gas

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13
Q

What happens to the internal energy of a gas and why, when a gas is compressed

A

Work is done on the gas by the surroundings and this increases the internal energy of the gas

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14
Q
A
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15
Q

What happens when a piston moves down on the cylinder containing a gas

A

It compresses the gas, work is done on the gas
The molecules are pushed closer together
Therefore, they have higher kinetic energy as they move faster
This increases the internal energy of the gas

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16
Q

What happens when a piston moves up a cylinder containing a gas

A

It expands the gas, work is done by the gas
The molecules are spread further apart
Therefore, they have lower kinetic energy as they move slower
This decreases the internal energy of the gas

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17
Q

How does heating the gas increase the internal energy by the same amount as doing work

A

Increasing the temperature of the gas means the molecules move around faster.
Therefore they have higher kinetic energy and increased internal energy

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18
Q

How does the number of molecules in the container change if the gas is expanding or contracting

A

IT DOESN’T. Number of molecules in the container ALWAYS stays the same

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19
Q

Specific heat capacity of a substance

A

The amount of thermal energy required to raise the temperature of 1kg of a substance by 1 degrees C without a change of state

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20
Q

Equation for specific heat capacity

A

Change in thermal energy = mass of substance being heated x specific heat capacity of substance x change in temp

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21
Q

How does heating and cooling of a substance vary with its specific heat capacity

A

Low specific heat capacity = Heats and cools down quickly as less energy needed to change its temp

High specific heat capacity = Heats and cools down slowly

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22
Q

What piece of equipment can be used to find the specific heat capacity of a fluid

A

A continuous-flow calorimeter

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23
Q

How does a continuous flow calorimeter work

A

A fluid flows continuously over a heating element where energy is transferred to the fluid.
It is assumed the heat transferred from apparatus to surroundings is constant

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24
Q

What variables are changed and kept constant in the continuous flow calorimeter experiment

A

Flow rate and pd is changed
Change in temperature of the fluid is constant

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25
Q

How to find mass of fluid in a continuous flow calorimeter

A

Record the flow rate
Multiple flow rate by the time taken to give the mass of the fluid that flows in as m1

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26
Q

How to find the electrical energy supplied to the fluid for the first flow rate in time t

A

current(1) x voltage(1) x time(1) = Q(1) = m(1) x c x change in temp + energy lost

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27
Q

What is the temperature change when a substance changes state

A

There is NO temperature change

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28
Q

Latent heat

A

The thermal energy required to change the state of 1kg of mass of a substance without any change of temperature

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29
Q

2 types of latent heat

A

Specific latent heat of fusion - melting a solid or freezing a liquid
Specific latent heat of vaporisation - vaporising a liquid or condensing a gas

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30
Q

What do the flat sections on a temp/heat supplied graph represent

A

The latent heats of ….

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31
Q

Equation for specific latent heat

A

Amount of thermal energy to change state = mass of substance changing state x latent heat of ….

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32
Q

Why is more energy needed to evaporate 1kg of water than to melt the same amount of ice into water

A

To melt ice, energy is required to just increase the molecule separation till they can flow freely over each other

To boil water, energy is needed to completely separate the molecules till there are no longer forces of attraction between the molecules.

33
Q

Why is latent heat of vaporisation of water much greater than specific latent heat of fusion of water

A

More energy has to be supplied to separate molecules than break a solid bond

34
Q

What energy changes take place during a change of state

A

Potential energies of the molecules change but NOT their kinetic energies

35
Q

What is kinetic energy proportional to

A

Temperature

36
Q

Absolute zero

A

The temperature at which the molecules in a substance have zero kinetic energy. It is the lowest temperature possible.

-273 degrees celscius

37
Q

How to convert celsius to Kelvin

A

Celsius = Kelvin - 273

38
Q

In the specific heat capacity equation, why is no unit needed for temperature

A

the difference in temperature between the 2 values will be exactly the same

39
Q

What is assumed in Boyles, Charles’ and Pressure Law

A

The mass and number of molecules of the gas are assumed to be constant for each of these laws

40
Q

Boyle’s Law

A

Pressure is inversely proportional to the volume of a gas
OR:
P1V1 = P2V2

ONLY IF TEMPERATURE OF IDEAL GAS IS CONSTANT

41
Q

Condition needed for Boyle’s Law

A

Temperature of the ideal gas must be constant

42
Q

Units in Boyle’s Law equation

A

Pressure - Pa
Volume - m^3

43
Q

What does a graph of Boyle’s Law of volume against pressure look like

A

Curve downwards, not touching either axis

44
Q

What would a Boyle’s Law graph look like if temperature is higher but still constant

A

The graph has the same shape but is shifted above the origin/ is higher up

45
Q

Charle’s Law

A

Volume is directly proportional to the temperature of a gas (V = kT)
OR:

V1/T1 = V2/T2

ONLY IF PRESSURE OF GAS IS CONSTANT

46
Q

Condition needed for Charles’ Law

A

Pressure of the ideal gas must be constant

47
Q

Units for temperature in Charles’ Law

A

Kelvin

48
Q

What does a graph of Charles’ Law look like for volume against temperature

A

Straight line through the origin. This shows direct proportionality

49
Q

What is an ideal gas

A

Obeys ideal gas law at all temperatures
Collisions of molecules are elastic
Molecules have negligible volume
No interactions between molecules

50
Q

How do molecules in a gas exert a pressure on a surface

A

Molecules move around randomly at high speeds, colliding with surfaces and exerting pressure on them.

51
Q

Describe what happens when the temperature of a gas is increased, to the pressure

A

The molecules move faster and so collide with the surface of the walls more frequently. Each collision applied a force across the SA of the walls. The faster the molecules hit the walls, the greater the force on the,. Therefore pressure increases

52
Q

What happens to the pressure when the volume of a box decreases and temperature stays constant and why

A

There will be a smaller surface area of the walls and hence more collisions. Therefore, pressure increases

53
Q

Pressure in an ideal gas

A

The force of collisions of the gas molecules per unit area of a container

54
Q

Ideal Gas equation (1)

A

pV = nRT

55
Q

Units in the ideal gas equation

A

Pressure = Pa
Volume = m^3
number of moles = moles
Temperature = Kelvin

56
Q

Ideal Gas equation (2)

A

pV = NkT
where N is the number of molecules and k is the Boltzmann constant

56
Q

What happens to the walls of a container when a gas expands

A

It does work on its surroundings by exerting pressure on the walls

57
Q

Work done when a volume of gas changes at constant pressure =

A

Pressure of gas x (increase in) Volume of gas

58
Q

Number of gas particles =

A

number of moles x Avogadros constant

59
Q

number of moles =

A

Mass of gas sample / molar mass

60
Q

mass =

A

molar mass / avogadros constant = mass of sample / number of gas particles

61
Q

Boltzmann constant =

A

Molar gas constant / Avogadros number

62
Q

Why is the value for Boltzmann constant very small

A

The increase in KE of a molecule is very small for every incremental increase in temperature

63
Q

What energies exist in ideal gas molecules

A

Only KE not potential energy as ideal gas molecules are assumed to have no intermolecular forces

64
Q

Change in internal energy of an ideal gas =

A

3/2 k x change in T

65
Q

Relationship between change in internal energy and change in temperature for ideal gas

A

Change in internal energy is proportional to change in temperature

66
Q

Assumptions in kinetic theory

A

Molecules of a gas behave as identical or have same mass
Molecules of gas are hard, perfectly elastic spheres
Volume of molecules is negligible compared to volume of container
Time of a collision is negligible compared to the time between collisions
There are no intermolecular forces between molecules except during impact
External forces such as gravity are ignored
Molecules move in continuous random motion
Newtons laws apply
There are a very large number of molecules
Elastic collision

67
Q

Why do we use average speed for molecules

A

Number of molecules of gas in a container is very large

68
Q

Elastic collision

A

No kinetic energy lost in collision

69
Q

Difference between gas laws and kinetic theory

A

gas laws are empirical in nature whereas the kinetic theory model arises from theory.

70
Q

Crms =

A

square root ( C1^2 + C2^2 + C3^2 C4^2 + …….) / N)

71
Q

Pressure in terms of Crms =

A

1/3 x m/V x N(Crms)^2

72
Q

When does Brownian motion of particles occur

A

When small particles suspended in a liquid or gas are observed to move around in a random, erratic fashion

73
Q

What does Brownian motion provide evidence for

A

The existence of atoms in a gas or liquids

74
Q

How to see Brownian motion

A

Under a microscope

75
Q

What does the random motion of particles (in Brownian motion) mean

A

A range of speeds
No preferred direction of movement

76
Q

How to find p2 using temp and intial pressure

A

P2 = T2 x P1 / T1

P1/T1 = P2/T2

77
Q

P1 x V1 at constant temp =

A

P2 x V2

AQA A Level Physics (15 decks)

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